By design, an MPTCP connection will not accept extra subflows where no
MPTCP listening sockets can accept such requests.

In other words, it means that if the 'server' listens on a specific
address / device, it cannot accept MP_JOIN sent to a different address /
device. Except if there is another MPTCP listening socket accepting
them.

This is what the new tests are validating:

 - Forcing a bind on the main v4/v6 address, and checking that MP_JOIN
   to announced addresses are not accepted.

 - Also forcing a bind on the main v4/v6 address, but before, another
   listening socket is created to accept additional subflows. Note that
   'mptcpize run nc -l' -- or something else only doing: socket(MPTCP),
   bind(<IP>), listen(0) -- would be enough, but here mptcp_connect is
   reused not to depend on another tool just for that.

 - Same as the previous one, but using v6 link-local addresses: this is
   a bit particular because it is required to specify the outgoing
   network interface when connecting to a link-local address announced
   by the other peer. When using the routing rules, this doesn't work
   (the outgoing interface is not known) ; but it does work with a
   'laminar' endpoint having a specified interface.

Note that extra small modifications are needed for these tests to work:

 - mptcp_connect's check_getpeername_connect() check should strip the
   specified interface when comparing addresses.

 - With IPv6 link-local addresses, it is required to wait for them to
   be ready (no longer in 'tentative' mode) before using them, otherwise
   the bind() will not be allowed.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/591
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 tools/testing/selftests/net/mptcp/mptcp_connect.c |  10 +-
 tools/testing/selftests/net/mptcp/mptcp_join.sh   | 153 +++++++++++++++++++++-
 2 files changed, 161 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/net/mptcp/mptcp_connect.c b/tools/testing/selftests/net/mptcp/mptcp_connect.c
index b148cadb96d0..c030b08a7195 100644
--- a/tools/testing/selftests/net/mptcp/mptcp_connect.c
+++ b/tools/testing/selftests/net/mptcp/mptcp_connect.c
@@ -1064,6 +1064,8 @@ static void check_getpeername_connect(int fd)
 	socklen_t salen = sizeof(ss);
 	char a[INET6_ADDRSTRLEN];
 	char b[INET6_ADDRSTRLEN];
+	const char *iface;
+	size_t len;
 
 	if (getpeername(fd, (struct sockaddr *)&ss, &salen) < 0) {
 		perror("getpeername");
@@ -1073,7 +1075,13 @@ static void check_getpeername_connect(int fd)
 	xgetnameinfo((struct sockaddr *)&ss, salen,
 		     a, sizeof(a), b, sizeof(b));
 
-	if (strcmp(cfg_host, a) || strcmp(cfg_port, b))
+	iface = strchr(cfg_host, '%');
+	if (iface)
+		len = iface - cfg_host;
+	else
+		len = strlen(cfg_host) + 1;
+
+	if (strncmp(cfg_host, a, len) || strcmp(cfg_port, b))
 		fprintf(stderr, "%s: %s vs %s, %s vs %s\n", __func__,
 			cfg_host, a, cfg_port, b);
 }
diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index c5169020a515..e323f81cdc02 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -62,6 +62,7 @@ unset sflags
 unset fastclose
 unset fullmesh
 unset speed
+unset bind_addr
 unset join_syn_rej
 unset join_csum_ns1
 unset join_csum_ns2
@@ -645,6 +646,27 @@ wait_mpj()
 	done
 }
 
+wait_ll_ready()
+{
+	local ns="${1}"
+
+	local i
+	for i in $(seq 50); do
+		ip -n "${ns}" -6 addr show scope link | grep "inet6 fe80" |
+			grep -qw "tentative" || break
+		sleep 0.1
+	done
+}
+
+get_ll_addr()
+{
+	local ns="${1}"
+	local iface="${2}"
+
+	ip -n "${ns}" -6 addr show dev "${iface}" scope link |
+		grep "inet6 fe80" | sed 's#.*\(fe80::.*\)/.*#\1#'
+}
+
 kill_events_pids()
 {
 	mptcp_lib_kill_wait $evts_ns1_pid
@@ -952,6 +974,7 @@ do_transfer()
 	local fastclose=${fastclose:-""}
 	local speed=${speed:-"fast"}
 	local in="${sin}"
+	local bind_addr=${bind_addr:-"::"}
 	port=$(get_port)
 
 	:> "$cout"
@@ -1005,7 +1028,7 @@ do_transfer()
 	timeout ${timeout_test} \
 		ip netns exec ${listener_ns} \
 			./mptcp_connect -t ${timeout_poll} -l -p ${port} -s ${srv_proto} \
-				${extra_srv_args} "::" < "${in}" > "${sout}" &
+				${extra_srv_args} "${bind_addr}" < "${in}" > "${sout}" &
 	local spid=$!
 
 	mptcp_lib_wait_local_port_listen "${listener_ns}" "${port}"
@@ -3230,6 +3253,133 @@ add_addr_ports_tests()
 	fi
 }
 
+bind_tests()
+{
+	# bind to one address should not allow extra subflows to other addresses
+	if reset "bind main address v4, no join v4"; then
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
+		bind_addr="10.0.1.1" \
+			run_tests $ns1 $ns2 10.0.1.1
+		join_syn_tx=1 \
+			chk_join_nr 0 0 0
+		chk_add_nr 1 1
+	fi
+
+	# bind to one address should not allow extra subflows to other addresses
+	if reset "bind main address v6, no join v6"; then
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 dead:beef:2::1 flags signal
+		bind_addr="dead:beef:1::1" \
+			run_tests $ns1 $ns2 dead:beef:1::1
+		join_syn_tx=1 \
+			chk_join_nr 0 0 0
+		chk_add_nr 1 1
+	fi
+
+	# multiple binds to allow extra subflows to other addresses
+	if reset "multiple bind to allow joins v4"; then
+		local extra_bind
+
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 10.0.2.1 flags signal
+
+		# Launching another app listening on a different address
+		# Note: it could be a totally different app, e.g. nc, socat, ...
+		ip netns exec ${ns1} ./mptcp_connect -l -p "$(get_port)" \
+			-s MPTCP 10.0.2.1 &
+		extra_bind=$!
+
+		bind_addr="10.0.1.1" \
+			run_tests $ns1 $ns2 10.0.1.1
+		chk_join_nr 1 1 1
+		chk_add_nr 1 1
+
+		kill ${extra_bind}
+	fi
+
+	# multiple binds to allow extra subflows to other addresses
+	if reset "multiple bind to allow joins v6"; then
+		local extra_bind
+
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 dead:beef:2::1 flags signal
+
+		# Launching another app listening on a different address
+		# Note: it could be a totally different app, e.g. nc, socat, ...
+		ip netns exec ${ns1} ./mptcp_connect -l -p "$(get_port)" \
+			-s MPTCP dead:beef:2::1 &
+		extra_bind=$!
+
+		bind_addr="dead:beef:1::1" \
+			run_tests $ns1 $ns2 dead:beef:1::1
+		chk_join_nr 1 1 1
+		chk_add_nr 1 1
+
+		kill ${extra_bind}
+	fi
+
+	# multiple binds to allow extra subflows to other addresses: v6 LL case
+	if reset "multiple bind to allow joins v6 link-local routing"; then
+		local extra_bind ns1ll1 ns1ll2
+
+		ns1ll1="$(get_ll_addr $ns1 ns1eth1)"
+		ns1ll2="$(get_ll_addr $ns1 ns1eth2)"
+
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 "${ns1ll2}" flags signal
+
+		wait_ll_ready $ns1 # to be able to bind
+		wait_ll_ready $ns2 # also needed to bind on the client side
+		ip netns exec ${ns1} ./mptcp_connect -l -p "$(get_port)" \
+			-s MPTCP "${ns1ll2}%ns1eth2" &
+		extra_bind=$!
+
+		bind_addr="${ns1ll1}%ns1eth1" \
+			run_tests $ns1 $ns2 "${ns1ll1}%ns2eth1"
+		# it is not possible to connect to the announced LL addr without
+		# specifying the outgoing interface.
+		join_connect_err=1 \
+			chk_join_nr 0 0 0
+		chk_add_nr 1 1
+
+		kill ${extra_bind}
+	fi
+
+	# multiple binds to allow extra subflows to v6 LL addresses: laminar
+	if reset "multiple bind to allow joins v6 link-local laminar" &&
+	   continue_if mptcp_lib_kallsyms_has "mptcp_pm_get_endp_laminar_max$"; then
+		local extra_bind ns1ll1 ns1ll2 ns2ll2
+
+		ns1ll1="$(get_ll_addr $ns1 ns1eth1)"
+		ns1ll2="$(get_ll_addr $ns1 ns1eth2)"
+		ns2ll2="$(get_ll_addr $ns2 ns2eth2)"
+
+		pm_nl_set_limits $ns1 0 2
+		pm_nl_set_limits $ns2 2 2
+		pm_nl_add_endpoint $ns1 "${ns1ll2}" flags signal
+		pm_nl_add_endpoint $ns2 "${ns2ll2}" flags laminar dev ns2eth2
+
+		wait_ll_ready $ns1 # to be able to bind
+		wait_ll_ready $ns2 # also needed to bind on the client side
+		ip netns exec ${ns1} ./mptcp_connect -l -p "$(get_port)" \
+			-s MPTCP "${ns1ll2}%ns1eth2" &
+		extra_bind=$!
+
+		bind_addr="${ns1ll1}%ns1eth1" \
+			run_tests $ns1 $ns2 "${ns1ll1}%ns2eth1"
+		chk_join_nr 1 1 1
+		chk_add_nr 1 1
+
+		kill ${extra_bind}
+	fi
+}
+
 syncookies_tests()
 {
 	# single subflow, syncookies
@@ -4184,6 +4334,7 @@ all_tests_sorted=(
 	M@mixed_tests
 	b@backup_tests
 	p@add_addr_ports_tests
+	B@bind_tests
 	k@syncookies_tests
 	S@checksum_tests
 	d@deny_join_id0_tests

-- 
2.51.0